[headless] Make browser service manifest overlay configurable.

headless/lib/embedder_mojo_browsertest.cc

Index: headless/lib/embedder_mojo_browsertest.cc
diff --git a/headless/lib/embedder_mojo_browsertest.cc b/headless/lib/embedder_mojo_browsertest.cc
index 512a8745c99256a21f7a0fde69826cf47613df69..5113d10fff6ed63b3466f066f6a09246f9344504 100644
--- a/headless/lib/embedder_mojo_browsertest.cc
+++ b/headless/lib/embedder_mojo_browsertest.cc
@@ -5,6 +5,7 @@
 #include <memory>
 #include "base/optional.h"
 #include "base/path_service.h"
+#include "base/strings/string_piece.h"
 #include "base/strings/stringprintf.h"
 #include "base/threading/thread_restrictions.h"
 #include "content/public/test/browser_test.h"
@@ -49,6 +50,15 @@ class EmbedderMojoTest : public HeadlessBrowserTest,
 
   ~EmbedderMojoTest() override {}
 
+  void SetUp() override {
+    // Set the manifest overlay before it is requested during main thread
+    // initialization.
+    options()->mojo_service_manifest_resource_id =
+        IDR_HEADLESS_EMBEDDER_TEST_MANIFEST_OVERLAY_JSON;
+
+    HeadlessBrowserTest::SetUp();
+  }
+
   void SetUpOnMainThread() override {
     base::ThreadRestrictions::SetIOAllowed(true);
     base::FilePath pak_path;